1. Technical Field
The present invention relates to a probe for providing a high speed connection, on the order of picoseconds, to a circuit or circuits on a wafer. In particular, the unique configuration of the probe allows for connection of the circuit on the wafer to other electrical systems including test equipment. The present invention provides a probe which permits picosecond speed/wideband measurements and diagnostics of the circuit on the wafer.
2. DESCRIPTION OF THE RELATED ART
At present, circuitry can be produced on wafers or chips by known methods. It is desirable to test the individual circuits on the wafer prior to packaging. Test equipment is known which is capable of operating at a speed in the picosecond range for carrying out tests on the individual circuits. It is necessary, however, to provide a connection from the test equipment to the circuits on the wafer or chip.
A means for providing a connection between test equipment and a circuit on a chip which is already cut from a wafer is disclosed in U.S. Pat. No. 3,596,228 to Reed, Jr. The patent discloses a fluid actuated contactor which provides multiple contacts between the circuit on chip and microstrip lines disposed on a flexible dielectric membrane. However, the microstrip lines on the membrane restrict the operating frequency band. Furthermore, the cumbersome structure makes it only suitable for making contacts to the circuit on a chip which is already cut from the wafer. It is not suitable for making contacts to circuits on the wafer, where the circuit density is very high.
U.S. Pat. No. 4,697,143 to Lockwood et al. discloses a wafer probe which is able to provide connections to circuits on wafer. However, this probe has the following disadvantages:
1. Lockwood's probe uses a thin ceramic for a substrate. The ceramic is easily broken and difficult to replace.
2. A large coaxial connection is used to be in direct contact with a transmission line on the probe. There is a 90.degree. bend at the interface of the coaxial cable and the transmission line. The sharp bend causes radiation, reflection and moding. Moreover, the configuration requires a probe of a large size which further degrades the RF (Radio Frequency) performance of the probe.
3. The large-size probe must be reduced to a small-size tip which contains small contacts. In order to make the reduction smooth and gradual, the length of the probe must be increased to about one inch. The long transmission line on the probe suffers excessive attenuation, dispersion, and parasitic coupling to the circuit under test. In a multi-channel configuration, cross talk between channels may also arise because of the long parallel transmission lines on the probe.
4. In order to avoid unwanted parasitic coupling between the probe and the circuit under test, as well as to avoid the cross-talk between different channels on the probe, absorbing materials and shielding are needed to make the probe into a multi-layer structure. In fact, the structure of the probe of U.S. Pat. No. 4,697,143 has seven layers. The complex structure makes the probe both costly and difficult to fabricate.
5. As a result of the above facts, the bandwidth and the speed of the probe are limited.
At present, a probe for on-wafer measurements is manufactured by Cascade and described in a brochure entitled "26.5 GHz Microwave Wafer Probing" by Cascad Microtech, Inc. Beaverton, Oregon. However, the probe has a maximum operational speed of 20 picoseconds. In addition there are other limitations to the operation of the probe which arise from the design of the probe. The design causes high reflection, radiation and high modes excitation especially at high speed or high frequency. This degrades the performance of any testing of the on-wafer circuit. In addition, the probe design results in the introduction of high insertion loss and cross talk while operating at high speeds and high frequencies. The construction of a substrate which is utilized to support the material which forms a conduit path in the probe suffers from a large discontinuity at the interface with the circuit on the wafer with which it connects. This is one of the causes of the reflection, radiation and high modes excitation which degrade the performance of the test circuitry at high speed and high frequency.
More specifically the limitations of the probe are set forth as follows.
The Cascade probe utilizes a coaxial to co-planar 90.degree. bend transition which causes high reflection, radiation and high modes excitation, especially at high speed/high frequency which thus degrades the performance of the system.
The Cascade probe utilizes a material having a high dielectric constant (such as alumina, E.sub.r =9) as the substrate for a co-planar line. This material causes a confinement of RF fields in the substrate. At the interface with the circuit on the chip the highly asymmetrical RF field distribution encounters a large discontinuity which causes reflection, radiation and high modes excitation resulting in a degradation of the high-speed/high frequency performance capabilities.
In order to obtain some mechanical flexibility, the alumina substrate utilized in the Cascade probe must be long (several centimeters). The long co-planar line disposed on the substrate suffers high dispersion, high insertion loss and cross talk, especially at high speed and high frequency. Therefore, further performance degradation is expected.
The contact tip of the Cascade probe is made of a very thin alumina chip which is fragile and can be easily broken during the testing process.
The Cascade probe's lack of sophistication prohibits it from using built-in printed circuit elements such as DC block/AC coupler, filter, mixer, electro-optical converter, and other electrical or optical networks for more complex tests.
The alumina utilized in the Cascade probe does not have the flexibility required for multi-channel independent contacts.
The end result is that the available probe limits the operational speed of the electrical circuitry, such as test equipment, which is to be connected to the on-chip circuit. The performance of the circuitry may also suffer degradation at high speeds and high frequencies. Finally, the known probe does not provide the sophistication and flexibility necessary for wide ranges of use.
As the operating frequency of semiconductor devices approaches higher and higher frequencies, a new probe with much better performance is needed to do on-wafer testing. The probe should achieve the following:
1. Flexibility: The probe should provide easy, secure and repeatable contact to the circuit under test.
2. Broadband and high performance: The probe should provide low insertion loss, low dispersion, low reflection, low radiation, and low cross-talk in a broadband up to 100 gigahertz.
3. simple and cost effective: The structure of the probe should be as simple as possible to make it easy to fabricate and the structure should be cost effective.
4. Small disturbances: For end-contact probes it is necessary to match the impedance of the circuit under test. Normally this requires 50 ohm impedance. When the probe is to be used for intermediate contact, a high impedance is required. It is necessary that all high speed/wideband probes have small parasitics.
5. High information flow: The probe should have the ability to operate in picoseconds in the time domain and must also have the capability of operating with a bandwidth of up to hundreds of gigahertz in the frequency domain.
6. versatility: The probe should be adaptable, that is it must be able to interface with different types of transmission lines having different dimensions.
7. Easy to use: The probe should easily make good contact with the circuit under test and it must easily interface with test instruments or other electronic, or electro-optical systems.
8. Rigidity and Durability: The probe should be rigid and should have the durability to make contacts thousands of times without performance degradation and with minimal physical decay.
9. Multi-channel test abilities: The probe should have the capability of testing complicated circuits. Therefore dual-channel and multi-channel configurations are required for providing input, output, DC supply and ground return signals among other signals.
10. On probe printed circuit: In order to test more sophisticated circuits a probe should include printed circuits such as DC block/AC coupler, filters, mixers, electro-optical converters, and other passive or active networks in order to enhance the operational capabilities of the device.